In a multi-antenna, linear-precoding system, a receiver selects an appropriate precoding matrix from a predefined set of precoding matrices in accordance with channel information, and returns an index of the selected precoding matrix in the set, just called as a codebook, to a transmitter. The transmitter then determines the corresponding precoding matrix in accordance with the received index, and performs appropriate pre-treatment on a to-be-transmitted signal using the precoding matrix, so as to improve validity and reliability of the information transmission. The codebook is a necessary element for the implementation of this procedure. The codebook needs to be designed in such a manner as to match channel distribution characteristics and minimize a performance loss due to codebook quantization, as possible. For 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and Institute of Electrical and Electronic Engineers (IEEE) 802.16 standards, different codebooks have been designed with respect to different antenna amounts, different antenna patterns and different application scenarios. However, these codebooks are designed with respect to a situation where the antennas are arranged in a one-dimensional manner, i.e., they are not suitable for the antenna array where the antennas are arranged in a two-dimensional manner.
A closed-loop precoding technique has been introduced into an LTE Release 8 (Rel-8) system, so as to improve the spectral efficiency. For the closed-loop precoding technique, it is at first required to store an identical set of precoding matrices, i.e., an identical codebook, at both a base station side and a User Equipment (UE) side. A UE estimates the channel information in accordance with a cell common pilot signal, and then selects one precoding matrix from the codebook in accordance with a certain criterion. The criterion for selection may include maximum mutual information and maximum output Signal-to-Interference plus Noise Ratio (SINR). The UE returns an index of the selected precoding matrix in the codebook to a base station via an uplink channel, and this index is just a Precoding Matrix Indicator (PMI). The base station may determine the precoding matrix to be used by the UE in accordance with the received index. The precoding matrix reported by the UE may be considered as a quantized value of Channel State Information (CSI).
In a conventional cellular system, the antenna array of the base station usually includes the antennas arranged horizontally, as shown in FIGS. 1 and 2. Beams from a transmitting end of the base station is merely capable of being adjusted in a horizontal direction, and for each UE, the beam has a fixed downtilt angle in a vertical direction, so various beamforming/precoding techniques are adopted on the basis of the channel information in the horizontal direction. Actually, a radio signal is transmitted in the space in a three-dimensional manner, and it is impossible for the fixed downtilt angle to achieve the optimal system performance. The adjustment of the beams in the vertical direction plays a very important role in the improvement of the system performance. Along with the development of the antenna techniques, an active antenna where each element is capable of being controlled separately has been presented in the industry, as shown in FIGS. 3 and 4. Through this kind of two-dimensional antenna array, it is possible to dynamically adjust the beams in the vertical direction. In a Frequency Division Duplexing (FDD) system, it is necessary for the UE to report the CSI so as to achieve the beamforming/precoding technique in a three-dimensional manner. A possible method includes reporting the CSI on the basis of the codebook, just like that always adopted by the LTE Rel-8 system. However, the existing codebooks are all designed with respect to the beamforming/precoding technique in the horizontal direction, and in the case that it is directly applied to the three-dimensional beamforming/precoding technique, the system performance may be degraded.
The codebooks reported on the basis of the CSI need to match the patterns and the application scenarios of the antenna array, so as to report the CSI in an accurate manner. In a scenario where the two-dimensional antenna array is adopted, there will be a very large number of possible patterns of the antenna array. In the case that a single codebook is to be designed to meet the requirements of all possible patterns of the antenna array, the design will be very difficult, or the resultant codebook may be very large.